Installing a second tank under the chassis is a major project. Nobody makes a kit for it, so you have to fabricate all your own mounts and connections. Most folks don't bother, and instead install the bigger single replacement fuel tank from TransferFlow:
Ford pickup replacement tanks
And if that 47 gallons is not enough, then your next step is to install a tank in the bed. A popular choice is a combo fuel tank/tool box. You'll find those on the TransferFlow website too:
Toolbox and fuel tank combos